Last year, Apple gave developers a slap on the wrists for using UDIDs—unique numbers attached to every iPhone which software designers like because they make it possible to keep track of who's using their apps. Now, however, it turns out that ad networks are bypassing a whole swathe of Apple's privacy rules using a host of new tricks.

"To avoid the limits of Apple's rules, ad networks that serve advertisements within mobile apps have started using new identifiers that collect information like location and preferences as the user moves across apps. One of the tracking systems is based on a unique identifier located in the iPhone's wireless networking hardware-a system known as Open Device Identification Number, or ODIN. The other prominent tracking alternative, called OpenUDID, uses the device's built-in copy-and-paste function."

The data which can be captured using these techniques allows marketeers to directly target ads to specific users. The ad networks that are using the tricks—such as SponsorPay—claim that the information only lets them "recognize users as a random set of numbers ", meaning they could never personally identify people using the data.

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However, privacy advocates are quick to point out that rolling a lot of this data—such as consumer habits, location and personal preferences—together could, over time, allow a company to link information back to a person's name. It's currently unclear what Apple will do about about the problem.